Problems with object orientation and angled array
Select two vertices, 
Change Transformation Orientations to Normal,    
Press the add button in the Transform Orientations panel then
Select new normal, ie Edge, in the drop down list.





In Object mode, shift+S > Cursor to selected,
Shift+A  > Empty > Plane Axes,
Set the constraint as shown in the figure and
Move Empty object using added normal(Edge).